1. Field of the Invention
The invention relates to a method of fabricating an SOI (silicon on insulator) substrate having a semiconductor active layer on an insulator, which substrate is considered to be promising as a substrate to be used for next generation LSI.
2. Description of the Related Art
One method of fabricating an SOI substrate having a semiconductor active layer on an insulator is suggested in Sadao Nakashima and Katsutoshi Izumi: "Analysis of buried oxide layer formation and mechanism of threading dislocation generation in the substoichiometric oxygen dose region", Journal of Material Research, Vol. 8, No. 3, pp 523-534, March 1993. The suggested method is the so-called SIMOX (Separation by Implanted Oxygen) technique which includes the steps of implanting oxygen ions (O+) with high doses into a silicon substrate, and applying thermal annealing at high temperature to the silicon substrate to form a continuous silicon dioxide (SiO.sub.2) film in the silicon substrate.
The SIMOX process has an advantage that an SOI substrate can be fabricated relatively readily, however, has a shortcoming that crystal defects tend to remain in an upper Si active layer in which semiconductor devices are to be fabricated. The density of the residual crystal defects is dependent on doses of implanted oxygen ions. In general, a higher dose causes a greater number of crystal defects.
Thus, it is necessary to lower the dose of implanted oxygen ion in order to decrease crystal defects. For instance, it is possible to decrease crystal defects by implanting oxygen ions at 180 KeV with doses of 3.times.10.sup.17 -4.times.10.sup.17 cm.sup.-2. However, if such low doses of oxygen ions are implanted into a silicon substrate, it is not possible to form a continuous silicon dioxide film even by subsequent thermal annealing at high temperature, thereby causing current leakage. Thus, desired characteristics of a semiconductor device cannot be obtained.
FIG. 1 schematically illustrates a silicon substrate into which low doses of oxygen ions are implanted according to prior art. At first, oxygen ions are implanted with low dosage into a silicon substrate 30, and then thermal annealing at high temperature is applied to the silicon substrate 30, thereby discontinuous dioxide islands 31 are produced in the silicon substrate 30, as illustrated in FIG. 1-B.
As having been described, the SIMOX process, which has a step of implanting oxygen ions (O+) with high dose into a silicon substrate, has an advantage that an SOI substrate can be fabricated relatively easily, but has a shortcoming that crystal defects tend to stay behind in an upper silicon active layer in which a semiconductor device is to be fabricated. On the other hand, if oxygen ions with low dose are implanted into a silicon substrate so as to decrease crystal defects, a continuous silicon dioxide film cannot be formed by subsequent thermal annealing at high temperature with the result of causing current leakage.